Why do birds need air bags?

Birds can have up to 5 air sacs in their body, which makes their breathing more efficient. The volume of the air sacs is ten times the volume of the lungs of the bird. These air sacs are a system that changes its volume as a result of inhalation and exhalation. The outgrowths of air sacs occupy a certain part of some bones of the skeleton of birds, which makes the skeleton more lightweight.
Air bags have the following meaning for birds:
First, air sacs are one of the body’s thermoregulation tools.
Secondly, these bags help aerate the lungs.
Thirdly, due to air sacs, the body density of birds changes during diving and swimming.